Rabble arm for a furnace

ABSTRACT

A rabble arm for a furnace, as e.g. a multiple hearth furnace, comprises an elongated metallic support core ( 10 ), at least one rabble tooth having a rabble portion and a fixing portion and fixing means co-operating with the fixing portion for fixing the rabble tooth ( 14 ) to the elongated metallic support core. The fixing portion ( 48 ) includes a through hole through ( 50 ) which said elongated metallic support core axially passes; and the fixing means co-operates with the fixing portion ( 48 ) around the through hole for fixing said rabble tooth ( 14 ) to said elongated metallic support core ( 10 ).

FIELD OF THE INVENTION

[0001] The present invention relates to a rabble arm for a furnace, inparticular a multiple hearth furnace.

BACKGROUND OF THE INVENTION

[0002] A multiple hearth furnace comprises an upright cylindricalfurnace housing that is divided by a plurality of vertically spacedhearth floors in vertically aligned hearth chambers. A vertical shaftextends centrally through the hearth chambers, passing through eachhearth floor. In each hearth chamber at least one rabble arm is fixed tothe vertical shaft and extends radially outside there-from over thehearth floor. Such a rabble arm is provided with rabble teeth, whichextend down into material being processed on the hearth floor. As thevertical shaft rotates, the rabble arm moves over the material on therespective hearth floor, wherein the rabble teeth plough through thematerial and mix the latter. Depending on the angle of inclination ofthe rabble teeth, the material will be moved radially inwardly towardthe vertical shaft or outwardly therefrom. Drop holes are provided ineach hearth floor, alternately in the inner zone of the hearth floor(i.e. near the vertical shaft) or in the outer zone of the hearth floor(i.e. near the cylindrical furnace housing). Material falling on theinner zone of a hearth floor is moved by the rabble arm radiallyoutwardly over this hearth floor, until it drops through a drop hole inthe outer zone of this hearth floor on the outer zone of a hearth floorlocated directly below. On this lower hearth floor, material is moved bythe rabble arm radially inwardly until it drops through a drop hole inthe inner zone of this hearth floor on the inner zone of the next lowerhearth floor. Thus, material to be processed is caused to move slowlyalong a serpentine path through the vertically aligned hearth chambersof the furnace.

[0003] It is a fact that multiple hearth furnaces possess majoradvantages over other solid material processing furnaces, such as rotaryhearth furnaces, rotary kiln furnaces and shaft furnaces. By allowing acontrol of different hearth atmospheres and temperatures in thevertically aligned hearth chambers, they allow a very close control ofthe process inside the furnace. Other advantages of multiple hearthfurnaces lie in their ability to maintain the processed materials inmixed condition throughout their passage through the furnace and towarrant a very intense exposure of the solid materials to process gasesin a controlled gas/solid material counter flow within the furnace.Nevertheless, since their invention at the end of the nineteenthcentury, multiple hearth furnaces have only found very few applicationsin solid material processing. A reason for this lack of confidence inmultiple hearth furnaces is that it has never been possible to warrant aproblem-free operation of a multiple hearth furnace over longer periods.

[0004] The most exposed elements in a multiple hearth furnace are therabble arms with their rabble teeth. These rabble arms and rabble teethare subjected to severe temperatures and severe mechanical constraintsin a furnace atmosphere that is usually very corrosive. Already in veryearly multiple hearth furnaces, the rabble arms included a water or gascooled cast iron support structure, and the rabble teeth were conceivedas exchangeable wear parts. Such an exchangeable rabble tooth generallyincludes a dovetail interlocking element at its upper portion engaging,in a form-fit relationship, a corresponding groove at the underside ofthe cooled metallic support structure.

[0005] An allegedly improved design of a rabble arm was disclosed in1968 in U.S. Pat. No. 3,419,254. This rabble arm includes a hollow castiron core obtained by mould casting. It is divided by a central web intotwo separate passageways for cooling air. The teeth of the arm areformed of a ceramic material. They have an upper fixing portion with apair of inwardly facing hook-like interlocking elements, which aredimensioned to fit loosely over lower horizontal flanges laterallyprotruding from the underside of the metallic core. In order to providean insulating and shock absorbing tight connection between the rabbleteeth and the metallic core, a fibrous insulating material is interposedbetween the hook-like formations and the lower horizontal flanges. Tocomplete the insulation of the metallic core, an inner layer of fibrousinsulation is placed over the top part of the metallic core, and anouter solid insulation is finally placed on top of the inner fibrousinsulation. Lugs on the metallic core prevent the cover from movinglongitudinally with respect to the metallic core. In an alternativeembodiment, a plurality of wire-like prongs is welded to the metalliccore along its sides and top. Thereafter, a layer of fibrous insulatingmaterial is pressed down over the prongs so that it lies snugly over thetop of the core. A castable insulation is finally cast over the exteriorof the rabble arm, where it is held in place by the wire-like prongs.

[0006] A first drawback of known rabble arms is a rather high frequencyof teeth breaks in the region of their dovetail or hook-like fixingportion. It will be noted in this context that a break-off of a singletooth may cause severe damages to the rabble arms of the hearth chamber,because the broken off rabble portion is an obstacle for the remainingrabble teeth and may cause a break-off of further teeth or even acollapse of whole rabble arms.

[0007] A further drawback of known rabble arms is their insufficientprotection against high temperatures. The thermal insulation of knownrabble arms is indeed deficient in respect of many aspects. It will benoted e.g. that the underside of the rabble arm, which is exposed to thehighest heat load, has the poorest insulation. Furthermore, it happensquite often that the thermal insulation of a rabble arm falls offalready after a short operation period of the furnace. As an overhaulingof the thermal insulation of a rabble arm requires the removal of therabble arm, the operator of the furnace usually runs usually the risknot to repair the thermal insulation of the rabble arms until the nextmajor overhauling of the furnace, which requires anyway the dismountingof the rabble arms. In the meantime, the unprotected metallic core ofthe rabble arm is however exposed to a much higher thermal load than thethermal load it is designed to withstand.

[0008] Still another drawback of present rabble arms is a poor wearresistance of their rabble teeth. Indeed, most rabble arms are stillequipped with cast iron rabble teeth, which become subject to rapid wearunder corrosive hearth atmospheres and/or high hearth temperatures.Ceramic rabble teeth would of course be more wear resistant in suchatmospheres, but the manufacture of ceramic form pieces of the size of arabble tooth is still a rather expensive operation. It follows that theuse ceramic rabble teeth is normally economically not justified.Furthermore, ceramic rabble teeth may be very wear resistant but theyhave nevertheless a low ductility, i.e. they are often subjected tobreakage in particular in the region of their dovetail or hook-likefixing portion.

OBJECT OF THE INVENTION

[0009] A first technical problem underlying the present invention is toprovide in a rabble arm for a furnace with an improved interconnectionbetween the elongated metallic support core and the rabble teeth. Thisproblem is solved by a rabble arm as claimed in claim 1.

SUMMARY OF THE INVENTION

[0010] Such a rabble arm for a furnace comprises an elongated metallicsupport core, at least one rabble tooth having a rabble portion and afixing portion and fixing means co-operating with the fixing portion forfixing the at least one rabble tooth to the elongated metallic supportcore. In accordance with an important aspect of the present inventionthe fixing portion does not include a dove tail or hook like fixingelement, but simply includes a through hole through which the elongatedmetallic support core axially passes, and wherein a fixing meansco-operates with the fixing portion around the through hole for fixingthe rabble tooth to the elongated metallic support core. It follows thatthe fixing portion of the rabble tooth does no longer include recessesthat generate stress concentrations that are probably responsible formost break-offs of rabble teeth. Furthermore, the fixing means canco-operate with the whole fixing portion around the through hole forfixing a rabble tooth to the elongated metallic support core. This meansthat—in comparison with a rabble tooth that is fixed by means of a dovetail or hook like fixing element—a better distribution of stresses canbe achieved in the fixing portion.

[0011] Another advantageous aspect of the new fixing portion is that theshape of the rabble tooth can be very simple. It may for example havethe form of a flat plate with an oval through hole. A direct consequenceof the sample shape of the rabble tooth is that it can for example bemade of a ceramic material at reasonable costs. In conclusion, thepresent invention allows to have at reasonable costs rabble teeth havinga good wear resistance and being far less subjected to break-off thanprior art rabble teeth.

[0012] The rabble tooth may have a constant thickness over its height.However, the thickness of the rabble tooth may also be varied over itsheight, so as to achieve a substantially uniform stress distribution inthe rabble tooth. It will be appreciated that such rabble tooth ofuniform strength has a reduced weight with regard to a rabble tooth witha constant thickness designed to resist to the same forces.

[0013] Instead of making the whole rabble tooth of a single material, itis also possible to conceive a rabble tooth having a fixing portion thatconsists of a first material and a rabble portion that consists of asecond material, wherein the first material is preferably more ductilethan the second material. In such a composite rabble tooth, the fixingportion has the advantage to have a good resistance against breakage (itdeforms plastically instead of breaking), whereas the rabble portion ismore wear resistant. It will be noted that the first material can forexample be a cast steel and the second material a ceramic material. Therabble tooth may also include a core made of cast steel, which extendsover the fixing portion and the rabble portion and is provided with aceramic jacket in the rabble portion.

[0014] In a preferred embodiment of the rabble arm, the fixing meanscomprises a teeth support sleeve slipped over the elongated metallicsupport core and engaging the through hole in the fixing portion of therabble tooth. Such a teeth support sleeve preferably supports severalrabble teeth by engaging their through holes. It provides advantageouslya form-fit with the elongated metallic support. In particular, the outercross-section of the teeth support sleeve and the through hole have forexample both an oval shape, so that the teeth support sleeve is blockedin rotation on the elongated metallic support core. The teeth supportsleeve provides advantageously a form-fit with the through hole of therabble tooth.

[0015] The teeth support sleeve may be thermally insulated, which allowsto have a continuous insulation of the elongated metallic support corethat is not interrupted by the fixing elements of the rabble teeth. Itwill be appreciated that such a continuously insulated teeth supportsleeve is thermally more efficient and moreover less exposed to afall-off than any prior art insulation of a rabble arm. Furthermore, anoverhauling of the thermal insulation of a rabble arm requires no longerthe removal of the rabble arm, the insulated teeth support sleeve can besimply slipped over the elongated metallic support core, therebyreplacing the rabble teeth and the thermal insulation in one operationfrom the outside of the furnace.

[0016] A preferred embodiment of such a teeth support sleeve includes aninner metallic sleeve and an outer metallic sleeve, an insulatingmaterial between the inner metallic sleeve and the outer metallic sleeveand a shock absorbing cushioning layer on the outer metallic sleeve. Thecushioning layer is engaged by the edge of the through hole in thefixing portion of the rabble tooth, whereby this edge is efficientlyprotected against mechanical damages, and a shock on one rabble tooth isabsorbed by the cushioning layer and not transmitted to the rest of theteeth support sleeve and the metallic support core. Furthermore, thecushioning layer helps to further improve the thermal insulation of therabble arm. It is recommended to make both the outer tube and the innertube of stainless steel. Such stainless steel tubes form an efficientcontinuous sheeting of the rabble arm against an excessive exposure tocorrosive gases.

[0017] A preferred embodiment of the teeth support sleeve furtherincludes armature elements protruding from the outer metallic sleevethrough the shock absorbing cushioning layer and a layer of castablerefractory on the shock absorbing cushioning layer, wherein the fixingportion of the rabble tooth is embedded in the refractory layer. It willbe appreciated that such a teeth support sleeve can easily be conceivedas a prefabricated unit to be simply slipped on the elongated metallicsupport.

[0018] A preferred embodiment of the metallic support core comprises twosuperimposed outer tubes, which are rigidly fixed together. Thesesuperimposed outer tubes are advantageously formed of centrifugally caststeel pipes. It will be appreciated that these outer tubes of themetallic support core have a very homogeneous structure that issubstantially free from casting cavities and other casting defects,which are unavoidable in a prior art support core obtained by mouldcasting. In conclusion, the metallic support core is-despite possiblylower manufacturing costs-less exposed to mechanical failures andcorrosion than any other metallic support core of prior art rabble arms.

[0019] In order to optimize cooling of the rabble arm, each of the outertubes advantageously includes a coaxial inner tube, which is arranged inthe outer tube so as to delimit therein an annular gap for a coolantflow. Thus, it is warranted to obtain an efficient and homogeneouscooling of the outer tubes. The cooling effect may further be improvedat reasonable costs, by simply arranging a wire is the aforementionedannular cooling gap, so as to define a spiral flow path for the coolantin the annular cooling gap.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present invention will now be described, by way of example,with reference to the accompanying drawings, in which:

[0021]FIG. 1A: is a longitudinal section through the rear end of arabble arm in accordance with the present invention;

[0022]FIG. 1B: is a longitudinal section through the front end of therabble arm of FIG. 1A;

[0023]FIG. 2: is a cross-section along section line 2-2 in FIG. 1A;

[0024]FIG. 3: is a longitudinal section through a teeth support sleeveof the rabble arm of FIG. 1;

[0025]FIG. 4: is a top view of the teeth support sleeve of FIG. 3;

[0026]FIG. 5: is a cross-section along section line 5-5 in FIG. 1A;

[0027]FIG. 6: is a front view of a rabble tooth; and

[0028]FIG. 7: is a vertical section through the rabble tooth of FIG. 6.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0029]FIGS. 1A and 1B show both end portions of an elongated rabble armin accordance with the present invention. This rabble arm is to besupported by a vertical shaft in a multiple hearth furnace. It includesan elongated metallic support core 10, i.e. a kind of hollow cantileverbeam that is fixed at one end with the help of a fixing flange 12 to thevertical shaft, so as to extend radially outside therefrom over a hearthfloor to the furnace wall. The object of this metallic support core 10is to support radially spaced rabble teeth 14 ₁, 14 ₂, 14 ₃, 14 ₄, whichextend down into the material on the hearth floor. As the vertical shaftin the hearth furnace rotates, the rabble arm moves over the material onthe respective hearth floor, wherein the rabble teeth 14 ₁, 14 ₂, 14 ₃,14 ₄ plough through the material on the hearth floor. Depending upon theangle of inclination of the rabble teeth 14 _(i) with respect to thelongitudinal axis of the rabble arm (see FIG. 4), the material will bemoved radially inwardly toward the vertical shaft or radially outwardlytherefrom.

[0030] The metallic support core 10 comprises two superimposed outertubes 16, 18, which are welded together (see in particular FIG. 2) andwelded at one end to the fixing flange 12 (see in particular FIG. 1A).Each of these tubes 16, 18 is preferably made up of one or morecentrifugally cast steel pipes. It will be appreciated that thecentrifugally cast steel pipes have a very homogeneous structure that issubstantially free from casting cavities and other casting defects,which are unavoidable in a prior art support core obtained by mouldcasting. In conclusion, the metallic support core 10 is—despite possiblylower manufacturing costs—less exposed to mechanical failures andcorrosion than any other metallic support core of prior art rabble arms.

[0031] Each of the outer tubes 16, 18 includes a coaxial inner tube 20,22, which is arranged in its outer tube 16, 18 so as to delimit thereinan annular gap 24, 26 for a coolant flow. A wire 28, 30 is arranged ineach of the annular gaps 24, 26 so as to define a spiral flow path forthe coolant. Through an inlet opening 32 in the flange 12 and an inletchamber 33 with a deflector plate 34, the coolant enters into theannular gap 24 of the upper tube 16 (see FIG. 1A), wherein it ischannelled in a spiral path along the inner wall of this tube 16 to theclosed front end of the latter (see FIG. 1B). Here the coolant passesthrough a communication opening 35 into the annular gap 26 of the lowertube 18, wherein it is channelled in a spiral path along the inner wallof this tube 18 to an outlet chamber 37 with a deflector plate 38 (seeFIG. 1A), which deflects the coolant into an outlet opening 39 in theflange 12. It remains to be noted that in most cases the coolant will bewater, but in specific cases it could be of interest to use a differentcooling fluid than water.

[0032]FIGS. 3 and 4 show a teeth support sleeve 40 supporting fourrabble teeth 14 ₁, 14 ₂, 14 ₃ and 14 ₄. This teeth support sleeve 40constitutes with its four rabble teeth 14 ₁, 14 ₂, 14 ₃ and 14 ₄ aprefabricated unit that is axially slipped on the elongated metallicsupport core 10. In FIG. 1A and FIG. 5, the teeth support sleeve 40 isshown in engagement with the elongated metallic support core 10. It willbe noted that such a teeth support sleeve 40 may have substantially thesame length as the elongated metallic support core 10, so that only oneteeth support sleeve 40 is to be slipped over the elongated metallicsupport core 10. However, for ease of handling, the teeth support sleeve40 will usually be substantially shorter than the elongated metallicsupport core 10, so that several teeth support sleeves 40 have to beslipped one after the other on the elongated metallic support core 10.It is to be understood that a teeth support sleeve 40 may of coursesupport more than four rabble teeth 14 or less than four rabble teeth14, and that it is also possible to conceive a “teeth” support sleevewith a single rabble tooth 14.

[0033] A preferred embodiment of the teeth support sleeve 40 includes aninner metallic sleeve 42 and an outer metallic sleeve 44, which are bothpreferably made of stainless steel. As shown on FIG. 5, the innermetallic sleeve 42 has an oval cross-section that provides a form-fitwith the elongated metallic support core 10. An insulating material 46,preferably a micro-porous insulating material, is arranged between theinner steel tube 42 and the outer metallic sleeve 44 to provide a goodthermal insulation.

[0034] A preferred embodiment of a rabble tooth 14 will now be describedwith reference to FIGS. 6 and 7. This rabble tooth 14 consists of a flatelongated ceramic plate, whose first end forms a rabble portion 46, andwhose second end forms a fixing portion 48. The fixing portion includesan ovally shaped through hole through 50 bounded by a rounded off orchamfered edge 52. This through hole 50 is more particularly shaped insuch a way that its edge 52 makes up a form-fit with the outer surfaceof the teeth support sleeve, when the rabble tooth 14 is in it isoperational position on the teeth support sleeve 40. It will be notedthat this outer surface of the teeth support sleeve 40 is advantageouslyformed by a thinner shock absorbing cushioning layer 54 that envelopsthe outer metallic sleeve 44. In summary, the elongated metallic supportcore 10 passes axially through the through hole 50 in the fixing portion48, and the teeth support sleeve 40 co-operates with the fixing portionaround the through hole 50 for fixing the rabble tooth 14 to theelongated metallic support core 10.

[0035] Because the rabble tooth 14 has the shape of a simple plate witha through hole in it, it can be manufactured at reasonable costs in aceramic material that has a good temperature and corrosion resistanceand an excellent wear resistance. Alternatively, only the rabble portion46 may be made of ceramic material, wherein the fixing portion 48 ismade of a cast steel. Such a composite rabble tooth has the advantagethat a cast steel is generally more ductile than a ceramic material andwill thus, under an excessive load, more likely plastically deformitself than break. It will be noted that a deformed rabble tooth may beineffective, but it does at least not fall on the hearth floor, where itwould present a risk for other teeth. In order to warrant a goodconnection between the rabble portion 46 and the fixing portion 48 in acomposite rabble tooth, the latter may include a core made of caststeel. This core extends over the rabble portion 46 and the fixingportion 48 and is provided with a ceramic jacket 55 in the rabbleportion. It is of course also possible to make the rabble tooth 14 ofany other material that has the required temperature, corrosion and wearresistant properties.

[0036] Referring again to FIGS. 3 and 4, it will be noted that thefixing portion 48 of the four rabble teeth 14 ₁, 14 ₂, 14 ₃ and 14 ₄ isembedded in a layer of castable refractory 60, which is cast around theshock absorbing cushioning layer 54. Wire-like armature elements 62 arewelded to the outer metallic sleeve 44, before the refractory 60 is castaround the shock absorbing cushioning layer 54. They protrude throughthe shock absorbing cushioning layer 54 to firmly anchor the refractory60 to the teeth support sleeve 40. In this way, the rabble teeth 14 ₁,14 ₂, 14 ₃ and 14 ₄ can be firmly blocked in their operating position onthe teeth support sleeve 40, wherein the forces acting on the rabbleportion are transmitted via the fixing portion 48 around the throughhole 50 as compressive forces onto the refractory 60 and via the edge 52of the through hole 50 and the shock absorbing cushioning layer 54 tothe teeth support sleeve 40. Additionally, metallic shouldering elements64 (see e.g. FIG. 4) may be welded to the to the outer metallic sleeve44, before the refractory 60 is cast around the shock absorbingcushioning layer 54. In this case the fixing portion 48 around thethrough hole 50 bears on these shouldering elements 64, so that thelatter contribute to the transmission of forces from the rabble teeth 14to the outer metallic sleeve 44. It will further be noted that metallicshouldering elements 64 also warrant that the rabble teeth aremaintained in their operating position even if the refractory 60 isdamaged or falls off.

[0037] Teeth support sleeves 40 as shown in FIGS. 3 and 4 can bemanufactured in a workshop ready for being slipped onto the metallicsupport core 10. As shown on FIG. 1B, the end of the last teeth supportsleeve 40′ (schematically indicated with a doted line) slipped onto themetallic support core 10 is secured to the latter by means of a pin 70.If the teeth 14 ₁, 14 ₂, 14 ₃ and 14 ₄ or the refractory 60 are worn outor damaged, then the teeth support sleeves 40 can be easily slipped offfrom the metallic support core 10 and replaced by new ones. Worn ordamaged teeth support sleeves 40 can be returned to a workshop for beingrefurbished under optimum conditions. It will be appreciated that theexchange of teeth support sleeves 40 can be easily effected from theoutside of the furnace through a maintenance door in the furnace wall,without having to dismount the metallic support core 10 or to enter thefurnace. On FIG. 5, reference numbers 72′ and 72″ refer to withdrawingrods arranged in a free space subsisting between the metallic supportcore 10 teeth support sleeves 40. These two withdrawing rods 72′ and 72″have one end engaged with the first teeth support sleeve 40 slipped ontothe metallic support core 10 and the other end protruding out of thelast teeth support sleeve 40′ slipped onto the metallic support core 10.They allow to easily slip off the teeth support sleeves 40 from themetallic support core 10.

1. A rabble arm for a furnace, said rabble arm comprising: an elongatedmetallic support core; at least one rabble tooth having a rabble portionand a fixing portion; fixing means co-operating with said fixing portionfor fixing said at least one rabble tooth to said elongated metallicsupport core; characterised in that said fixing portion includes athrough hole through which said elongated metallic support core axiallypasses; and said fixing means co-operates with said fixing portionaround said through hole for fixing said rabble tooth to said elongatedmetallic support core.
 2. The rabble arm as claimed in claim 1,characterised in that said through hole is ovally shaped.
 3. The rabblearm as claimed in claim 1 or 2, characterised in that said rabble toothhas the form of a flat plate.
 4. The rabble arm as claimed in claim 3,characterised in that said rabble tooth is made of a ceramic material.5. The rabble arm as claimed in claim 4, characterised in that saidfixing portion consists of a first material, and said rabble portionconsists of a second material, wherein said first material is moreductile than said second material.
 6. The rabble arm as claimed in claim5, characterised in that said first material is a cast steel and saidsecond material is a ceramic material.
 7. The rabble arm as claimed inany one of claims 1 to 3, characterised in that said rabble toothincludes a core made of cast steel, said core extending over said fixingportion and said rabble portion and being provided with a ceramic jacketin said rabble portion.
 8. The rabble arm as claimed in any one ofclaims 1 to 7, characterised in that said fixing means comprises a teethsupport sleeve slipped over said elongated metallic support core andengaging said through hole in said fixing portion of the rabble tooth.9. The rabble arm as claimed in claim 8, characterised in that saidteeth support sleeve supports several rabble teeth by engaging theirthrough holes.
 10. The rabble arm as claimed in claim 8 or 9,characterised in that said teeth support sleeve provides a form-fit withsaid elongated metallic support.
 11. The rabble arm as claimed in anyone of claims 8 to 10, characterised in that said teeth support sleeveprovides a form fit with said through hole of said at least one rabbletooth.
 12. The rabble arm as claimed in claim 11, characterised in thatthe outer cross-section of said teeth support sleeve and said throughhole are both ovally shaped.
 13. The rabble arm as claimed in any one ofclaims 8 to 11, characterised in that said teeth support sleeveincludes: an inner metallic sleeve and an outer metallic sleeve; aninsulating material between said inner metallic sleeve and said outermetallic sleeve; and a shock absorbing cushioning layer on said outermetallic sleeve, said cushioning layer being engaged by the edge of saidthrough hole in the fixing portion of said rabble tooth.
 14. The rabblearm as claimed in claim 12, characterised in that said outer tube andsaid inner tube are both made of stainless steel.
 15. The rabble arm asclaimed in claim 13 or 14, characterised in that said teeth supportsleeve further includes: armature elements protruding from said outermetallic sleeve through said shock absorbing cushioning layer; and alayer of castable refractory on said shock absorbing cushioning layer,wherein said fixing portion of said at least one rabble tooth isembedded in said refractory layer.
 16. The rabble arm as claimed inclaim 15, characterised in that said teeth support sleeve forms aprefabricated unit, which includes said at least one rabble tooth andsaid layer of castable refractory, and said prefabricated unit isdesigned to be slipped on said elongated metallic support.
 17. Therabble arm as claimed in any one of claims 8 to 16, characterised inthat said teeth support sleeve includes metallic shouldering elements,wherein said fixing portion bears on said shouldering elements.
 18. Therabble arm as claimed in any one of claims 1 to 17, characterised inthat said metallic support core comprises two superimposed outer tubes,which are rigidly fixed together.
 19. The rabble arm as claimed in claim17, characterised in that said superimposed outer tubes are made ofcentrifugally cast steel pipes.
 20. The rabble arm as claimed in claim18 or 19, characterised in that each of said outer tubes includes acoaxial inner tube, which is arranged in said outer tube so as todelimit therein an annular gap 24, 26 for a coolant flow.
 21. The rabblearm as claimed in claim 20, characterised by a wire that is arranged issaid annular gap so as to define a spiral flow path for the coolant insaid annular cooling gap.
 22. A multiple hearth furnace comprising atleast one rabble arm as claimed hereinbefore.